For expository convenience, the present invention is described with reference to one particular application thereof, namely a system for recovering digital subcarrier data from a conventional FM broadcast signal. It should be recognized, however, that the invention is not so limited.
Subcarriers on FM broadcast signals are increasingly being used to transmit digital data to subscribers of subcarrier data services. Data being transmitted by such services includes stock market reports and paging information. A subcarrier-based paging system is disclosed in U.S. Pat. No. 4,713,808 to Gaskill, the disclosure of which is incorporated herein by reference.
It is well known that a continuous signal must be sampled at a frequency above the Nyquist rate if the signal is to be properly characterized. (The Nyquist rate is defined as twice the signal's highest frequency.) If a sub-Nyquist rate is used, frequency aliasing results, causing various portions of the signal's spectrum to interfere with each other. If this interference is uncontrolled, the signal can be lost or scrambled. That is, the sampled data may correspond to two or more different input signals. To avoid this possibility, most digital systems sample at rates well in excess of the Nyquist rate.
In many applications, it is desirable to sample a signal at less than the Nyquist rate. By so doing, the system's cost and power consumption are reduced, and its hardware complexity is simplified. One such application is the paging system described in the Gaskill patent referenced above, in which the receiver is implemented in wristwatch form.
In the modulation system originally described in the Gaskill patent, the sampling problem was avoided by using phase shift keying. The data modulating the subcarrier was recovered at the wristwatch receiver by noting whether the subcarrier was in phase or out of phase with the pilot signal. This detection of the subcarrier phase was performed at a 19 KHz rate.
While the phase shift keying method was advantageous in certain respects, it required a subcarrier bandwidth of 38 KHz to achieve a 19 Kbit transmission rate--an inefficient use of spectrum. Modulation of the subcarrier using amplitude modulation would have permitted more efficient use of the spectrum, but would have required sampling at a rate in excess of the Nyquist criteria--a feat difficult to achieve given the constraints associated with the wristwatch design.
To permit use of an amplitude modulated subcarrier in the Gaskill system, it is an object of the present invention to circumvent the Nyquist sampling criteria. More particularly, it is an object of the present invention to permit a digital receiver to sample an amplitude modulated subcarrier at sub-Nyquist rates in a manner that controls the frequency aliasing such that the aliasing produces a constructive interference between the upper and lower sidebands.
Briefly, this object is achieved in the present invention by sampling the amplitude modulated subcarrier synchronously to its carrier. By sampling the signal at the times when the subcarrier is at its peak values, the data signal can be recovered at less than the Nyquist rate. Sampling is synchronized to the subcarrier by generating a sampling clock from a synchronizing signal transmitted with the modulated carrier that is phase locked thereto.
The foregoing and additional objects, features and advantages of the present invention will be more readily apparent from the following detailed description thereof, which proceeds with reference to the accompanying drawings.